The present invention relates to the general art of surgery, and to the particular field of introducing material to a patient for therapeutic or diagnostic purposes, most specifically, the invention relates to NO therapy.
The use of therapeutic gases to treat a human or animal patient has been known in the art for many years. A number of different gases may be added to a respiratory gas that is inhaled by a spontaneously breathing, non-ventilated patient. These gases may be used to achieve some therapeutic effect, service a diagnostic function or have some other desirable purpose. Such gases will be referred to herein as xe2x80x9ctherapeutic gases.xe2x80x9d One skilled in the delivery of therapeutic gas will understand that the disclosure can be used to teach either human or animal patients. Accordingly, no limitation to human is intended by references to patient in this disclosure.
One therapeutic gas is nitric oxide (NO), which is administered by inhalation in low concentrations to treat primary or secondary pulmonary hypertension or other diseases. In many cases, nitric oxide or other therapeutic gases come from a high concentration source such as a high concentration compressed gas cylinder. The gas source may be pure or may contain some concentration of therapeutic gas in a carrier gas. There may also be cases where more than one therapeutic gas is used, with or without a carrier gas or gases. It is often necessary to dilute therapeutic gas to a lower concentration and mix it with air and/or oxygen prior to delivery to the patient. This dilution may be necessary to achieve a desired dosage concentration and/or to avoid or reduce adverse bioeffects that may occur if high concentration gas is delivered to the patient. If the therapeutic/carrier gas is not sufficiently oxygenated, it is necessary to mix it with air prior to delivery to the patient. In some cases, it is necessary to add supplemental oxygen to the mixture to avoid a hypoxic respiratory mixture or to enrich the oxygen content of the respiratory gas above twenty-one percent. In the latter case, the oxygen will also be considered as a therapeutic gas.
NO is one of a number of therapeutic gases that are administered to a patient and require dilution from a high concentration form to a lower, safer concentration before administration to a patient. No will be the primary focus of this disclosure; however, one skilled in the surgical arts will understand that the disclosure can be used to teach other gases as well. Accordingly, no limitation to NO is intended by the references to NO in this description.
The art contains several devices and systems to deliver therapeutic gas to a spontaneously breathing, non-ventilated patient. However, as will be discussed, each of the known systems and devices has drawbacks.
A system that has continuous flow to a mask is one such known system. A therapeutic gas, oxygen and air are supplied from sources such as compressed gas cylinders or a hospital wall. A continuous flow of these gases is titrated together before delivery to a patient. The flow rate of each gas is set to achieve the desired concentration of the therapeutic gas and oxygen in the respiratory gas. The total flow rate is set greater than the peak inspiratory flow rate. If a reservoir bag is added to the inspiratory portion of the overall circuit, then the total flow can be reduced, but must still be greater than the minute volume of the patient. The mixed gas is connected into the mask, from which the patient inhales. Exhaled gas and excess inhalation gas flow from an outlet side of the mask and may be scavenged. This system has the disadvantage of wasting gas since not all therapeutic gas is inhaled by the patient. Scavenging is required to prevent the therapeutic gas from entering the environment. In addition, large volumes of air and/or oxygen must be supplied to dilute/mix the therapeutic gas. Also, therapeutic gas is delivered to the entire respiratory tract, not just the areas where it is needed. This may increase adverse bioeffects and the possibility of undesirable reaction products from the therapeutic gas. The mask also makes eating and talking difficult and is also aesthetically unappealing. Still further, a mask may make some patients nervous and cause anxiety by making them feel confined.
Yet another system uses a bolus pulse of therapeutic gas to a mask. In this system, therapeutic gas is delivered to the patient as a bolus of gas that is delivered via the mask. The bolus of therapeutic gas is delivered over a short period of time and is not significantly diluted by inhaled air or supplemental oxygen. Supplemental oxygen may also be delivered via the mask. The patient""s breathing waveform is monitored and the bolus of therapeutic gas is delivered to the mask intermittently, in synchronization with the respiratory waveform so that the therapeutic gas is inhaled at a set phase of the respiratory waveform. The bolus is preceded and/or followed into the respiratory tract by air/oxygen. This system and method has the disadvantage that it does not dilute the therapeutic gas, so a high concentration source cannot be used. In addition, the short duration of the bolus means that a higher concentration of therapeutic gas is required to deliver the same number of molecules of the gas to the patient. This could have adverse bioeffects. This method does not have the flexibility of varying the concentration of the therapeutic gas at various times during inspiration. The mask has the same drawbacks as heretofore discussed.
Yet another system and method uses an undiluted pulse via a nasal cannula. A nasal cannula is a device that can be used to transmit therapeutic gas from one or more therapeutic gas sources to the nose of a patient for inhalation. It includes one or more connectors at one end of the device to connect to one or more therapeutic gas sources, one or more long lumens to transmit the gas, and nasal prongs at the other end to inject one or more therapeutic gases into the patient""s nose. The word xe2x80x9clumenxe2x80x9d will be used in this disclosure to represent a long, narrow, flexible fluid conduit that is less than 0.8 cm in internal diameter. A nasal cannula is typically much less obtrusive than a mask and allows the patient to talk and eat while receiving gas therapy. In the method of undiluted pulse delivery via a nasal cannula, therapeutic gas is delivered via a nasal cannula as an intermittent flow pulse during inspiration. Air pressure in the nares drops at the start of inspiration. This pressure drop is transmitted through the cannula and is detected in the pulse delivery device. Therapeutic gas flow is turned on for a period of time during inspiration. The therapeutic gas flows directly into the nares from the cannula. While overcoming many disadvantages associated with a mask, this method also has disadvantages as practiced in the known art. For example, the therapeutic gas is not diluted prior to entering the nares in many known systems. If a high concentration source is used, high concentration gas may contact the tissues before it is diluted in the respiratory tract. This may have adverse bioeffects. If lower concentration gas is used, the source lifetime/size advantages of a high concentration source are lost. Also, the final dilution concentration in the respiratory tract is limited. It is lower for any given volume of therapeutic/carrier gas, and this volume must be limited to avoid a hypoxic respiratory gas mixture. Still further disadvantages will be discussed below in reference to the use of known cannulas.
Still another known method and system for administering therapeutic gas to a patient includes an undiluted pulse via a nasal cannula and oxygen via another lumen. In this method, gas may be delivered as discussed above, with the addition of supplemental oxygen delivered via a second lumen in a dual lumen cannula. This method has all the disadvantages discussed above, except that it allows a higher diluted concentration to be delivered to the respiratory tract without having a hypoxic mixture. This has the accompanying disadvantage of requiring a supplemental oxygen source.
A diluted pulse to a cannula can also be used. In this method, the therapeutic gas may be delivered by a nasal cannula and diluted prior to entering the nares. This can be done by mixing it with a diluent gas from a diluent gas source before it leaves the cannula. The therapeutic gas concentration can be reduced to a safe level prior to entering the nares. It is further diluted in the respiratory tract by entrained air from the room. This method has the disadvantage of requiring a diluent gas source. If supplemental oxygen therapy is desired, oxygen or enriched air may be used as the diluent gas, but it is more difficult to control the oxygen concentration reaching the respiratory tract because a minimum diluent gas flow is required to dilute the therapeutic gas to a safe concentration in the cannula.
Still another known method includes a continuous flow of therapeutic gas and supplemental oxygen delivered to the patient via a nasal cannula. This method has therapeutic gas delivered continuously via a nasal cannula by titrating the therapeutic gas with air and/or oxygen before the cannula or in the cannula before it reaches the nares. The therapeutic gas concentration can be reduced to a safe level prior to entering the nares. It is further diluted in the respiratory tract by entrained air from the room, This method has the disadvantage of requiring a diluent gas source. If supplemental oxygen is required, a source of air and a source of oxygen will be required or it will be difficult to control the oxygen concentration reaching the respiratory tract.
Yet another known method of administering therapeutic gas to a patient includes use of a transtracheal catheter. In this method, therapeutic gas can be delivered directly to the trachea of the patient via a transtracheal catheter. Therapeutic gas flow might be continuous or pulsed. This method has the disadvantage that the therapeutic gas is not diluted prior to entering the respiratory tract. If a high concentration source is used, high concentration gas may contact the tissues before it is diluted in the respiratory tract. This may have adverse bioeffects. If lower concentration gas is used, the source lifetime/size advantages of a high concentration source are lost. Also, the final diluted concentration in the respiratory tract is limited. It is lower for any given volume of therapeutic/carrier gas and this volume must be limited to avoid a hypoxic respiratory gas mixture. The transtracheal catheter is invasive, which is often undesirable.
The art has also developed methods which deliver therapeutic gas to a patient during certain times. In such systems, gas delivery is pulsed on during inspiration. Other systems also include means for adjusting dosages, durations, flow rates and the like.
It is noted that not every patient has the same breathing pattern as other patients so a pulse configuration and time that is suitable for one patient may not be completely efficient for another patient. The shape of the gas pulse (flow rate versus time profile) of a first gas may be an approximately arbitrary shape. Some devices for pulsed gas delivery to spontaneously breathing patients use a pulse of a set flow rate and vary the duration of the pulse to change the dosage of gas to a patient. This results in an approximately rectangular flow versus time shape of the pulse. Other devices use a constant pulse duration but flow rate is altered to change dosage. Flow rate is constant during any single pulse and the pulse shape is approximately rectangular.
There is a need for a system that is adaptable to customizing the pulse shape, can easily adjust the dose, is adaptable to various conditions and modes of operation for various patients having individual requirements and is easily maintained by various caregivers.
Still further, some patients require delivery of more than one therapeutic gas. Therefore, there is a need for a system that is amenable to delivering more than one therapeutic gas to a patient if necessary.
Since many patients have individual requirements, it is necessary that a therapeutic gas delivery system be amenable to use by a variety of caregivers ranging in expertise from professional nurses and doctors to laymen in a home environment. In order to be most efficient and effective, the system should efficiently deliver therapeutic gas to the patient at all desired times, even if a primary source of gas is being changed. This may be particularly important in some applications such as nitric oxide therapy where interruption of the therapy can result in a xe2x80x9creboundxe2x80x9d effect where patient symptoms become as bad as or worse than they were before the therapy began. To be most versatile, the system should be amenable to use with either a nasal cannula (nasal prongs) or a mask and be easily used, monitored and maintained by a variety of caregivers.
There is thus a need for a system which is amenable to use by a variety of caregivers and which has means for delivering therapeutic gas in an uninterrupted manner when desired.
More specifically, even though there are several cannulas known in the art, these known cannulas have various drawbacks that may vitiate advantages obtained from customizing a therapeutic gas delivery system in order to overcome the drawbacks associated with known systems.
For example, known cannulas do not have means for efficiently controlling mixing of gases and do not have a gas mixing area that is most efficient or most efficiently located.
Therefore, there is need to improve the cannulas now used in connection with therapeutic gas administering systems.
More specifically, many known cannulas do not provide a location for mixing gases that is remote from a patient""s nares. Such a remote mixing location can be advantageous for better control of the final mixture administered to the patient. Such a remote location can also be controlled without inhibiting a patient in any way. However, since known cannulas do not have such an element, they have disadvantages.
Still further, many known cannulas have designs that waste therapeutic gas. Further, many known cannulas cannot be used in a system that can precisely detect breathing patterns of a patient and cannot be used to precisely and accurately control dosage, concentration and flow rates of the gases.
Therefore, there is a need for a cannula that efficiently administers therapeutic gas to a patient in a manner that overcomes the drawbacks of known cannulas.
Still further, many systems that are used to administer therapeutic gas to a patient include primary gas sources in the form of pressurized cylinders. Some of these systems include a xe2x80x9cflow directionxe2x80x9d check valve downstream of the inlet to seal the system when the supply pressure is removed. However, a check valve system may have drawbacks if used in certain circumstances.
For example, when a pressurized source is exchanged, there exists the possibility that air will be trapped within the volume of the system plumbing that is exposed to air during the exchange. It is desirable to keep that volume of air as small as possible so the resulting trapped air volume is reduced. Any trapped air will degrade the quality of the high purity gases contained within the remainder of the system when intervening valves are opened. This degradation is proportional to the volume of trapped air. Therefore, it is desirable to maintain this dead volume to a minimum.
Furthermore, it is advantageous to provide a system sealing action as close to the supply inlet as possible to further minimize the dead space volume upstream of the sealing surfaces. A flow direction check valve is not able to achieve all of these goals. Therefore, there is a need for an equalizing valve that can minimize dead space volume.
It is noted that it is possible to flush or purge the system to remove contaminated gas from some dead space regions. However, for purging to be effective, the dead space must be substantially swept out during periods of gas flow. If there are poorly swept regions within the dead space, purging will have to be extended to allow for diffusion and other mechanisms to dilute the contaminated regions. Therefore, there is a need for a means for ensuring proper purging of a system used to administer therapeutic gas to a patient.
Furthermore, purging requirements are strongly dependent on the relative size of the contaminated volumes. Purging is often complicated in many situations due to possible toxic effects of the therapeutic gases; and the high cost of medical grade gases.
Therefore there is a further need for a valve that will make purging most efficient and effective while overcoming the just-mentioned problems.
It is also noted that an autonomous gas delivery system should be able to detect the supply pressure so when a pressurized cylinder has been attached and the supply valve opened, a control system is signaled. This requires suitable positioning of a pressure sensing element.
However, in order to maintain low dead space, a pressure sensor must be located on the downstream side of an inlet sealing mechanism. In the prior art, a simple back flow prevention check valve has provided this function. A check valve will seal when there is a lower supply pressure on the downstream side of the check valve. If the check valve seals, the pressure sensor, which is located further downstream of the check valve, will continue to show the pressure when the check valve is closed and will not indicate the actual supply pressure. If, subsequent to this, a supply is attached that is at a lower pressure than the xe2x80x9ccheckedxe2x80x9d pressure, the system will not be able to detect the connection.
Therefore, there is a need for a means for sealing a system such as disclosed herein which will be able to fully detect and control the flow of the system during changing of gas sources.
In general, it is desirable to close off the inlet of a system such as disclosed herein when a supply is detached and to maintain the inside of the high purity system at a positive pressure with respect to atmospheric pressure. By closing off the inlet, the chance of contamination is reduced. By maintaining a positive internal pressure, any small leaks that may be present will tend to leak in an outward fashion helping to prevent atomospheric gas from entering the system.
Therefore, there is a need for a means for connecting the system of the present invention to a source of gas that will reduce the possibility of contamination of the system.
It is a main object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient.
It is another object of the present invention to manage NO delivery to a spontaneously breathing, non-ventilated patient such that concentrated NO is as low as reasonably possible while delivering the desired amount of NO to the distal portions of the patient""s lungs.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient that is compatible with periodic, routine or continuous modes of operation.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient that is easily used by patients, clinical staff and other caregivers with a wide and varying range of skills.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient that is easily cleaned, purged and maintained.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient that is easily monitored.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient that has any limited lifetime elements thereof easily replaced.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which will minimize concentration of therapeutic gases delivered to any tissue that requires treatment.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which will accurately and efficiently deliver a desired concentration and dose to the patient.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which can be adapted for use with a cannula or a mask while still accurately and efficiently delivering desired doses and concentrations of therapeutic gases to the patient.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which can deliver any desired therapeutic gas or combination of gases to the patient in an efficient and effective manner.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which has alarms and alarm systems.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which includes locks to prevent undesired operation of the system or its elements.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which has means for providing system operational history.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which is adaptable to a wide variety of conditions and system requirements.
It is another object of the present invention to provide a system and elements for delivering No to a spontaneously breathing, non-ventilated patient which has means for delivering desired therapeutic gases even while a main source of gas is being replaced.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which is portable.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which is autonomous.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which can respond to changes in patient parameters such as breath rate and tidal volume.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which can respond to changes in environmental parameters.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which has a low transit time of gases through an entrainment cell.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which has an entrainment cell that is easily cleaned and maintained.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which has an entrainment cell that is not prone to clogging.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which has an entrainment cell that efficiently mixes gases.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which has an entrainment cell that can monitor flow rates.
It is another object of the present invention to provide a system and elements for delivering No to a spontaneously breathing, non-ventilated patient which has an entrainment cell that is spaced from a patient""s face.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which has an entrainment cell with an efficient and effective geometry.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which has an entrainment cell with an entrainment cell that can be efficiently flushed during operation.
It is another object of the present invention to provide a system and elements for delivering No to a spontaneously breathing, non-ventilated patient which has an entrainment cell that is easily inspected.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which has an entrainment cell that has a total flow rate of gases during inspiration which is a large fraction of a patient""s inspiratory flow rate.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which has an entrainment cell with a low dwell time of therapeutic gases as compared to the desired delivery flow rate.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient which has a means for effectively and efficiently equalizing pressure between a source of pressurized gas and the system, but keeping the system pressurized slightly above atmospheric pressure if the gas source is removed.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient that maintains dead volume to a minimum
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient that includes an equalizing valve that can minimize dead space volume
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient that includes a means for ensuring proper purging of a system used to administer therapeutic gas to the patient.
It is another object of the present invention to provide a system and elements for delivering NO to a spontaneously breathing, non-ventilated patient that includes a valve that will make purging most efficient and effective while overcoming the problems associated with the prior art.
It is another object of the present invention to provide a system and elements for delivering No to a spontaneously breathing, non-ventilated patient that includes a means for sealing a system such as disclosed herein which will be able to fully detect and control the flow of the system during changing of gas sources.
It is another object of the present invention to provide a system and elements for delivering No to a spontaneously breathing, non-ventilated patient that includes a means for connecting the system to a source of gas that will reduce the possibility of contamination of the system.
These, and other, objects are achieved by a system that administers therapeutic gas to a spontaneously breathing, non-ventilated patient which accurately manages and supervises the delivery of gas. The system dilutes a high concentration therapeutic gas to a lower concentration prior to delivering it to a patient for inhalation. The system delivers the gas via nasal prongs or via a mask. It effects this delivery without requiring a source of pressurized diluting gas such as from a pump or compressed gas cylinder. Dilution can be accomplished with room air of close to local atmospheric pressure. The system also allows supplemental oxygen to be added to the respiratory gases if desired and works with a pulsed delivery device.
The system also includes a special cannula that further improves the overall effectiveness of the system. The cannula has an entrainment cell that is sized and shaped to produce a gas transit time therethrough that is most effective to properly mix, dilute and deliver the therapeutic gas to the patient. The entrainment cell is also sized and shaped to inhibit clogging and can be transparent if desired to provide a visual indication of the inside of the cell. The entrainment cell has ports and other elements that are located and positioned to thoroughly mix the gas with room air in a manner that is most effective and efficient. Furthermore, one form of the entrainment cell includes flow sensors that can be used so gas administration is accurate and precise. The flow sensor can act as an interlock to help ensure that sufficient room air is entrained for adequate dilution. Other forms of the cell include check valves, baffles and other ports. The entrainment cell provides desired mixing control that is suitable for the accurate system and is located remote from the patient""s nares whereby patient safety and comfort are enhanced. The cell also is closed except for the ports and thus gas is not wasted due to leakage. Flow sensors can also be used to transmit pressure signals from the patient""s nose through the therapeutic gas lumen so the beginning of inspiration may be detected.
The entrainment cell can be used with a cannula or with a mask as desired.
The system of the present invention can deliver one or more therapeutic gases to a patient for inhalation and can dilute the therapeutic gas with room air prior to delivering the gas to the patient. The room air for dilution is entrained by the respiratory effort of the patient so no supplementary air source such as compressed air or an air pump is required. The total flow rate of gases in the device during inspiration, including entrained air, is a large fraction of the patient""s inspiratory flow rate and the total flow rate of the gases in the device is equal to the sum of the flow rate of each therapeutic gas plus the flow rate of entrained air. One form of the invention delivers a low flow rate of nitric oxide during inspiration where the nitric oxide flow rate is very small compared to the inspiratory flow rate. In such a case, the flow rate of entrained air is a large fraction of the patient""s inspiratory flow rate. The high fraction of inspired air flowing through the device is achieved in part by the geometry and size of an entrainment cell and associated elements. The geometry gives a low flow resistance in the air entrainment port, entrainment cell, outlet lumen and nasal prongs used in conjunction with the entrainment cell. The size of the nasal prongs is a factor in the resistance to flow around the prongs and into the patient""s nose. A form of the invention that includes a mask allows an even greater fraction of the inspired gas to travel through the device.
The entrainment cell is small, lightweight and relatively unobtrusive and the dwell time of therapeutic gas in the cell is low compared to the desired delivery flow rate because of the size of the entrainment cell. One form of the device includes a check valve at the outlet of the therapeutic gas lumens and these lumens are usually small to limit the gas conductivity thereat. The entrainment cell also ensures proper turbulent mixing of the gases prior to delivery to the patient. The cell can also include a narrowing of flow paths near the outlet of the entrainment cell to increase the turbulence for improving mixing as gas leaves the cell.
One important performance characteristic of the therapeutic gas delivery system is its temporal response. The temporal response of the system depends partially on its geometry, one aspect of which is the mixing location for the therapeutic gas. In one embodiment, the therapeutic gas is mixed near the inlet end of the entrainment cell. In another embodiment, the therapeutic gas is mixed between a baffle in the entrainment cell and the outlet end of the cell. In yet another embodiment, the therapeutic gas is mixed near the patient. In connection with this, we define several temporal characteristics.
Cannula latency Tc is the delay from therapeutic gas metering from the gas controller until the therapeutic gas reaches the patient.
Therapeutic lumen propagation latency Tp is the delay for a flow rate to propagate from the metering valve in the gas controller to the outlet of the therapeutic gas lumen. This delay is very small in all cases relevant to the disclosed system and will be neglected.
Cell inlet latency Tci is the delay for a therapeutic gas to travel from a mixing region near the inlet end of the entrainment cell to the outlet lumen.
Cell baffle latency Tcb is the delay for a therapeutic gas to travel from a mixing region between the baffle and the outlet end of the entrainment cell to the outlet lumen.
Near patient mixing latency Tnp is the delay for a therapeutic gas to travel from a mixing region near the patient to the patient.
Outlet lumen latency To is the delay for gas to travel from the beginning of the outlet lumen at the entrainment cell to the patient.
The Cannula latency will depend on where the therapeutic gas is injected from the therapeutic gas lumen. If therapeutic gas is injected at the inlet end of the entrainment cell, then
Tc≈Tci+To
If therapeutic gas is injected between the baffle and the outlet end of the entrainment cell, then
Tc≈Tcb+To
If therapeutic gas is injected near the patient, then
Tc≈Tnp
Where approximations have been made because the therapeutic lumen propagation latency has been neglected.
A small cannula latency is established using the present invention. The cell inlet latency and cell baffle latency are made small by having a small entrainment cell internal volume compared to the volume of gas flowing through the cannula during inspiration. The outlet lumen latency is made small by keeping the outlet lumen internal volume small compared to the volume of gas flowing through the cannula during inspiration. The near patient mixing latency may be made arbitrarily small by making the mixing location as close to the patient as desired.
One form of the entrainment cell has an air inlet port located in an end wall thereof. This location of the air inlet port helps prevent blockage of the port. The location of a therapeutic gas lumen next to the air inlet port and parallel to the cell axis also helps prevent blockage.
One form of the entrainment cell is also designed to be flushed out with each patient breath. A combination of a small internal volume and flow design achieves this.
The entrainment cell is easy to clean as it has a fairly simple internal structure. The cell in one embodiment is transparent for easy inspection. One form of the entrainment cell includes a flow sensor that can be used to accurately ensure that sufficient air is entrained to properly dilute the therapeutic gas as well as to measure the inspiratory flow of the patient and to detect the beginning of inspiration. The flow sensor can also be used to control the pulse rate and size of the gas delivery system.
The small size of the entrainment cell reduces the low pass filtering effect (provides a better transient response). The entrainment cell transmits the waveform of the therapeutic gas to the patient with minimal distortion other than dilution. The flow rate of therapuetic gas injected from the therapeutic gas lumen may vary with time. The same waveform of therapeutic gas is delivered to the patient along with the room air and any other therapeutic gas with the system of the present invention. A low pass filtering effect exists due to the particular geometrical characteristics of the device such as the small diameter and the length of the gas lumens and the volume of the entrainment cell. A small entrainment cell volume and narrow lumens reduce the filtering effect.
Some forms of the entrainment cell include check valves on the air entrainment port or ports to prevent therapeutic gas from escaping from the cell even if the therapeutic gas flow rate is large. Thus, the desired amount of therapeutic gas reaches the patient.
Some forms of the entrainment cell have interlocks for connecting the cell to the remainder of the flow circuit.
The present invention also includes an equalizing valve that equalizes pressure in low dead volume conditions. The equalizing valve simultaneously satisfies a number of objectives and overcomes the problems associated with the prior art as discussed above.
The equalizing valve of the present invention satisfies the above-stated objects. The valve has inlet sealing surfaces within the valve fitting that engages a supply fitting of a source of pressurized gas at the closest possible location to the supply inlet. The remaining volume of the inlet is reduced by substantially filling that volume with a pin, leaving a thin annulus for gas to pass into the system. This geometry helps preserve the downstream gas purity and will significantly reduce the required amount of purge gas.
The valve of the present invention maintains a sufficient positive internal pressure to ensure that air does not migrate into the high purity gas regions. Furthermore, gas is not allowed to enter the high purity regions until a sufficiently high supply pressure is attached to the system. As an added safety feature, the valve of the present invention permits flow to automatically throttle itself at very high rates in the event of a massive leak.